How cold is cold and how north is north? A location may help on suggestions. In western WA, I do not heat coops, and in matter of fact, most my birds choose to sleep "outdoors" and not in their hutches all winter. Mostly mild winters in the 30's and 40's, but even last year when temps were in the teens for a few weeks and with a foot of snow outside, only the old birds and the small birds went inside.
Most importantly, make sure there is ventilation and humidity does not get high in the coop. They can deal with single digits no problem, but just below freezing and damp air will give them frost bite.